Metal-polymer composites comprising nanostructures and applications thereof

ABSTRACT

Metal-polymer composites, and methods of making and use thereof, said composites comprising a thermally-cured dense polyaniline substrate; an acid dopant; and, metal nanostructure deposits wherein the deposits have a morphology dependent upon the acid dopant.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. patent applicationSer. No. 11/653,004, filed Jan. 11, 2007.

STATEMENT REGARDING FEDERAL RIGHTS

This invention was made with government support under Contract No.DE-AC52-06NA25396 awarded by the U.S. Department of Energy. Thegovernment has certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates to metal-polyaniline composites, e.g.,nanostructured metal-polyaniline composites, and to methods ofpreparation thereof.

BACKGROUND OF THE INVENTION

In the past few decades, there has been an increasing interest in usingconducting polymers to fabricate electronic or optical devices such aslight-emitting diodes and molecular electronics. Polyaniline (PANI) hasbeen one of the most commonly used conducting polymers due to its facilesynthesis, low cost and environmental stability, and has been carefullyexamined for use in electromagnetic shielding and anticorrosioncoatings.

PANI possesses secondary and tertiary amines in the backbone structurethat can reversibly bind metal ions. In cases where metal ions have areduction potential higher than that of the PANI, the bound metal ionscan be reduced to form zero-valent metals. Thus, it has been recognizedthat PANI can be used for the electrodeless deposition of metals from ametal ion solution.

When suitable acid dopants are used during the deposition of metals ontoa surface, unique nanostructures may be formed which correlate to thetype of acid dopant. Surprisingly, it was found that these metalnanostructure-PANI composites were useful substrates for SERS (SurfaceEnhanced Raman Spectoscopy), and provided not only significantlyincreased sensitivity, but an inexpensive alternative to availablesubstrates. Previous work employed an asymmetric, porous PANI substrate.It is desirable, however, to also be able to deposit metalnanostructures onto thermally-treated, dense PANI films, as such filmsare more easily obtained by thermally evaporating the solvent.

Previously, it was not thought that nanostructures could be successfullyproduced on dense film substrates. As described in U.S. patentapplication Ser. No. 11/653,004, the use of PANI porous asymmetricmembranes for the chemical deposition of metal layers, e.g.,nanostructured metal layers has an advantage of lower density, ascompared to thermally cured dense PANI films. The lower density isthought to allow diffusion of metal ions between PANI chains, which mayaid the nucleation process. Furthermore, the phase inversion processused to prepare the PANI membranes does not involve heat treatment for aprolonged time period. The phase inversion process allows PANI tomaintain its original redox states and is believed to minimizecrosslinking between PANI chains.

Previous attempts at growing metal nanostructures on non-poroussubstrates resulted in different metal morphologies as compared to thosegrown on porous asymmetric membranes. The growth of metals on thethermally cured dense films exhibited less variation in theirmorphologies. In general, silver growth on top of a series of thermallycured PANI dense films exhibited large microstructures with randommorphologies. After further research, however, it has surprisingly beenfound that metal nanostructures having reproducible morphologies can beproduced on non-porous substrates which are useful for a variety ofapplications, including SERS.

SUMMARY OF THE INVENTION

The present invention describes thermally treated, dense PANI filmshaving a variety of nanostructures on the surface, and methods forcontrollably depositing the nanostructures.

The following describe some non-limiting embodiments of the presentinvention.

According to a first embodiment of the present invention, a method offorming a metal-polymer composite is provided, comprising providing athermally-cured dense polyaniline substrate; contacting thethermally-cured dense polyaniline substrate with an acid dopant to forma doped substrate; providing a solution comprising a metal ion; andcontacting the solution comprising a metal ion with the doped substrate,whereupon metal nanostructure deposits are formed on the dopedsubstrate, wherein the deposits have a morphology which is dependentupon the acid dopant.

According to another embodiment of the present invention, a method offorming a metal-polymer composite is provided, comprising providing athermally-cured dense polyaniline substrate; providing a solutioncomprising a metal ion and an acid dopant; and contacting the solutioncomprising the metal ion and acid dopant with the thermally-cured densepolyaniline substrate, whereupon metal nanostructure deposits are formedon the doped substrate, wherein the deposits have a morphology which isdependent upon the acid dopant.

According to yet another embodiment of the present invention, a methodof performing surface-enhanced Raman spectral analysis of an analyte isprovided, comprising providing a a thermally-cured dense polyanilinesubstrate, wherein a surface of the substrate comprises metalnanostructure deposits and an acid dopant; bringing an analyte intoeffective contact with said metal-PANI composite; illuminating themetal-PANI composite with radiation of at least one wavelength forcausing elemental-state particles to produce a plasmon field and forcausing said plasmon field to interact with molecules of said analyte,in such a manner, so as to produce Raman photons; and, collectingscatted radiation emitted from said metal-PANI composite for spectralanalysis.

According to yet another embodiment of the present invention, ametal-polymer composite is provided, comprising a thermally-cured densepolyaniline substrate; an acid dopant; and, metal nanostructure depositswherein the deposits have a morphology dependent upon the acid dopant.

BRIEF DESCRIPTION OF THE DRAWINGS

In all Figures, the x-axis represents wavelength in cm⁻¹ and the y-axisrepresents signal intensity.

FIGS. 1( a)-(b) show a SERS spectrum of mercaptobenzoic acid (MBA) (a)obtained from a silver nanostructure of the present invention made with1 mM silver nitrate and doped with citric acid, and the correspondingSEM image of the nanostructure (b).

FIGS. 2( a)-(b) show a SERS spectrum of mercaptobenzoic acid (MBA) (a)obtained from a silver nanostructure of the present invention made with10 mM silver nitrate, and the corresponding SEM image of thenanostructure (b). PANT dense film shown in (a); and, (d) the silvercoated HCl doped PANT dense film shown in of the nanostructure (b).

FIGS. 3( a)-(b) show a SERS spectrum of mercaptobenzoic acid (MBA) (a)obtained from a silver nanostructure of the present invention made with20 mM silver nitrate and doped with citric acid, and the correspondingSEM image of the nanostructure (b).

FIGS. 4( a)-(b) show a SERS spectrum of mercaptobenzoic acid (MBA) (a)obtained from a silver nanostructure of the present invention made with100 mM silver nitrate and doped with citric acid, and the correspondingSEM image of the nanostructure (b).

FIGS. 5 (a)-(b) show a SERS spectrum of mercaptobenzoic acid (MBA) (a)obtained from a silver nanostructure of the present invention made with20 mM silver nitrate and doped with tartaric acid, and the correspondingSEM image of the nanostructure (b).

FIGS. 6 (a)-(b) show a SERS spectrum of mercaptobenzoic acid (MBA) (a)obtained from a silver nanostructure of the present invention made with20 mM silver nitrate and doped with phosphoric acid, and thecorresponding SEM image of the nanostructure (b).

FIGS. 7 (a)-(b) show a SERS spectrum of mercaptobenzoic acid (MBA) (a)obtained from a silver nanostructure of the present invention made with20 mM silver nitrate and doped with camphosulfonic acid, and thecorresponding SEM image of the nanostructure (b).

FIGS. 8 (a)-(b) show a SERS spectrum of mercaptobenzoic acid (MBA) (a)obtained from a silver nanostructure of the present invention made with20 mM silver nitrate and doped with mandelic acid, and the correspondingSEM image of the nanostructure (b).

FIGS. 9 (a)-(b) show a SERS spectrum of mercaptobenzoic acid (MBA) (a)obtained from a silver nanostructure of the present invention made with20 mM silver nitrate and doped with p-toluene sulfonic acid, and thecorresponding SEM image of the nanostructure (b).

FIGS. 10 (a)-(b) show a SERS spectrum of mercaptobenzoic acid (MBA) (a)obtained from a silver nanostructure of the present invention made with20 mM silver nitrate and doped with polystyrene sulfonic acid, and thecorresponding SEM image of the nanostructure (b).

FIGS. 11( a)-(b) show a SERS spectrum of mercaptobenzoic acid (MBA) (a)obtained from a gold nanostructure of the present invention made with200 mM AuCl₃ with HCl, and doped with citric acid, and the correspondingSEM image of the nanostructure (b).

DETAILED DESCRIPTION

The present invention is concerned with the metal-polyaniline compositescomprising a dense-film PAM substrate, and to methods of producing thesemetal-polyaniline composites, wherein the metal forms variousnanostructures on the surface of the composite. The present invention isalso concerned with the use of such metal-polyaniline composites, e.g.,such nanostructured metal-polyaniline composites, in applicationsemploying Surface Enhanced Raman Scattering (SERS).

“Thermally-cured dense polyaniline substrate,” as used herein, means asubstrate made according to a method comprising the steps of spreadingthe polymer solution onto a solid surface into a wet film and heating toa specified temperature, and is understood not to include polyanilinesubstrates that are formed and/or combined with a metal ion in solution.

“Morphology dependent upon the acid dopant,” as used herein, means thata particular acid dopant reproducibly results in a specific morphologywhen applied under conditions specified herein with respect to aciddopant concentration, metal concentration, identity of the metal, timeof immersion, etc.

The substrates of the present invention are dense-film substrates, whichare comprised of a single, substantially uniform layer of a conductivepolymer such as PANI. In contrast to asymmetric porous PANI membranes,which are produced by a solvent exchange process, production of thesubstrates of the present invention requires thermal treatment. Thesubstrates of the present invention are understood not to includesubstrates formed by mixing a metal ion and a polymer in solution, butrather, the metal is grown in situ after formation of the substrate.

In the present invention, metal-polyaniline composites are prepared bygrowing the metal particles on the substrate in situ, as opposed todepositing pre-formed metal structures onto a substrate. In contrast tocurrently available SERS substrates, which are small in size andexpensive to produce, the method of the present invention is suitablefor inexpensively producing composites that in theory may be of any sizeranging from a square millimeters to square meters. The composites maybe prepared by electrodeless deposition. The reduction potential of PANIat its emeraldine base form versus a standard reference hydrogenelectrode is from about 0.70 V to 0.75 V whereas PANI at itsleucoemeraldine base form versus a standard reference hydrogen electrodeis from about 0.30 V to 0.40 V. Where the particular metal ions have areduction potential greater than that of the PANI, immersion of asuitable PANI film or membrane into an aqueous solution of theparticular metal ions can result in the deposition of metal upon thePANI and form the metal-PANI composite. For silver nitrate (reductionpotential of 0.8 V), gold chloride (reduction potential of 1.5 V), andplatinum chloride (reduction potential of 0.755 V), a metal coatingincluding metal nanoparticles can form upon a PANI porous asymmetricmembrane or thermally cured PANI dense film dipped into the aqueousmetal salt solution. Metals useful in the composite structures of thepresent invention can generally include metals such as gold, silver,platinum, palladium and the like.

Conducting polymers suitable for use in the present invention includeconducting polymers such as polyaniline (PANI), polypyrrole,polythiophene, poly(phenylene vinylene), and combinations thereof. Inone embodiment, the conducting polymer is polyaniline.

Polyaniline is the name given to the polymer having the structure, in acompletely reduced leucoemeraldine oxidation state, of the generalformula:

where n is greater than 5 and where R is a hydrogen atom. Alternatively,R may be a substituent such as an organic group, including, for example,CH₃, C₂H₅, OCH₃, N(CH₃)₂, an inorganic group, including, for example, F,Cl, Br, I, or a metal chelate group. For the polyanilines describedherein, the appropriate choice of an R group permits a greater range ofsolubility in a greater number of different types of solvents, whichresults in increased versatility for processing the polymers and agreater range of chemical properties.

In situ deposition of metals on top of PANI surfaces can be achieved byimmersing thermally cured dense PANI films into an aqueous metal ionsolution with a wide range of metal ion concentrations. In oneembodiment, the metal ion concentration ranges from about 0.001 Molar(M) to about 1.0 M. The time period necessary for the metal growthranges from about several seconds to about several hours depending uponthe experimental conditions such as temperature and metal ionconcentration. The membranes are typically left in solution until metaldeposition can be observed visually on the target surface.

In one embodiment of the present invention, the aqueous solutioncomprising the metal is first doped with a suitable acid, and a suitablesubstrate is then immersed in the aqueous solution. In an alternativeembodiment, the substrate is provided and doped with a suitable acid.The doped membranes are then immersed into the aqueous solutioncomprising the metal. The morphology and size of the nanostructures isdependent upon, and correlates to, the identity of the acid dopant.Suitable acid dopants include, but are not limited to, hydrochloric acid(HCl), tartaric acid, camphosulfonic acid, phosphoric acid (H₃PO₄),trifluoroacetic acid (CF₃COOH), citric acid(2-hydroxypropane-1,2,3-tricarboxylic acid), mandelic acid(2-phenyl-2-hydroxyacetic acid), poly(styrene sulfonic acid),para-toluenesulfonic acid (p-CH₃C₆H₅SO₃H), and combinations thereof. Theresulting deposited metal nanostructures exhibited varying morphologiesas seen in FIGS. 1-11.

The morpohologies of the nanostructures include sheets, cubes, spheres(e.g., sphere-like dots, as shown in FIGS. 1, 5 and 6), needles (e.g.,needle-like structures, as shown in FIG. 2), rods (e.g., rod-likestructures as shown in FIGS. 3 and 4), wires (e.g. as shown in FIG. 9),and random nanostructures (e.g., as shown in FIG. 11).

Metal deposition on the PANT membrane surfaces through reductionrequires electron transfer from the PANI to the metal ions. As the metalions approach the PANI membrane surface, they can be reduced by the PANIand form nuclei. In a conventional homogeneous system, where metal ionsand a reducing agent are both dispersed in solution, the metal nucleican serve as catalytic sites for subsequent surface growth allowing theformation of larger metal structures. The present invention involves aheterogeneous system. In such a heterogeneous system, metal ions arereduced on the PANT membrane surface itself, and the growth mechanismcan be expected to differ from a homogeneous system. While not wishingto be bound by the present explanation, the size of the metalnanoparticles is likely to be dominated by the difference in reductionpotential between metal ions and the PANI and by the surface propertiesof the PANT membrane substrate. The morphological differencecorresponding to the various dopants may be influenced by the surfaceenergy of the PANI membrane. The surface energy of the PANI membrane,manifested by the water contact angles, can be tuned by the nature ofthe dopants and the redox states of the PANI membrane

Control of metal growth could also be achieved by using a stretched PANIfilm (a thermally cured PANI dense film). The stretching of the PANIdense films can generally be from about 100 percent to 500 percent ofthe original size, more preferably from about 100 percent to about 400percent of the original size. The contrast between silver deposited onthe stretched and unstretched PANI dense film is shown in FIG. 7. Apreferential deposition of the silver on the PANI dense film surfacecoincided with the stretched direction. In contrast, the unstretchedPANI dense films exhibited homogeneous silver deposition throughout thePANI dense film surface. Without wishing to be limited by theory, such apreferential silver deposition is thought to result from the lowerreduction potential of the stretched oriented PANI dense film. Thestretched (extended) PNI chain conformation leads to formation ofcrystalline regions having lower reduction potential and higherconductivity that the unstretched film portions.

The metal-PANT composites of the present invention can provide goodcatalytic activity to various organic reactions especially towardhydrogenation reactions. As the metal, e.g., metal nanoparticles, arestably adfixed to the PANT membrane, it can be easily recycled andreused whereas typical commercially available metal catalysts, e.g., Pdon C, can only be used once.

In one embodiment, metal-PAM composites of the present invention whereinthe metal is platinum or palladium can be used in the selectivehydrogenation of alkenes and alkynes.

Surface roughness has been considered an important factor that governsSERS. It has been previously observed that molecules near roughenedsilver surfaces show enhanced Raman scattering of as much as two ordersof magnitude or more. The ability to detect and identify trace amountsof chemicals has become increasingly important. SERS has proven to beone of the most sensitive methods for performing chemical analysis bythe detection of a single molecule. SERS offers the advantage of tracedetection by increasing the scattering light intensity by as much as afactor of 10¹⁰, especially with nanostructured metals such as silver andgold.

The SERS effect leads to a major increase of the Raman scatteringcross-section for molecules absorbed onto suitably rough metal surfaces.Thus, use of the composites of the present invention in SERSapplications has a large potential in analytical chemistry and forbiological applications. The structures of the present invention resultin an enhancement of the localized electric field, which subsequentlyincreases the SERS from the metal particles in the composite structures.

Through the process of the present invention, the formation of metals onreductive PANI surfaces can be controlled by tailoring the surfacechemistry and redox potential. A wide range of metal nanostructuremorphologies can be achieved by selection of dopant. The sizes of thedeposited metal structures could also be varied from a few nanometers toseveral microns. The composite materials of the present invention mayfurther have application in surface processing, catalysis (e.g., in fuelcells or other chemical reactions), as modified surface electrodes, foranticorrosion and for biosensors.

The present invention is more particularly described in the followingexamples which are intended as illustrative only.

EXAMPLES

Reagents: N-Methyl-2-pyrrolidone (99% Aldrich), heptamethylenimine(HPMI, 98% Acros), citric acid (99.9% Fisher), silver nitrate (99.9999%Aldrich), hydrochloric acid (37%, Fisher), gold chloride (99%, Aldrich),4-mercaptobenzoic acid (90%, Aldrich), and polyaniline emeraldine base(EB) powder (Aldrich) were used as received.Microscopy: PANI membrane surfaces were imaged using either a JEOL6300FXV SEM or a FEI Quanta FEG ESEM.Spectroscopy: All metallic substrates were prepared for spectroscopicanalysis by forming a self assembled monolayer of 4-mercaptobenzoic acid(MBA) on their surface. Each substrate was immersed in a 5% ethanolicsolution of MBA for 1 hour and then rinsed with fresh ethanol threetimes and allowed to air dry. Surface enhanced Raman spectra (SERS) wererecorded using approximately 1 mW, 785 nm excitation focused onto thesample through a 0.5 NA microscope objective. The scattered Raman signalwas collected in a backscattering configuration through the objective,filtered, and then dispersed onto a liquid nitrogen cooled CCD camerathrough a single grating spectrometer. All spectra were recorded usingtotal integration times of 10 seconds.PANI Starting Solution: Concentrated. PANI solutions (20% w/w) wereprepared as follows. First, 1.15 g of EB powder was mixed with 0.747 gof heptamethyleneimine (HPMI) in a 12 mL Teflon vial for ten minuteswith a spatula. Then, 4.14 g of N-methyl-2-pyrrolidinone (NMP) was addedand stirred with a spatula for 30 minutes until the visible solid PANIparticles were dispersed. The PANI solution was then filtered through a10 mL syringe using a cotton plug.PANI Thin Films: The concentrated PANI solution was poured onto a glasssubstrate and smeared into a homogeneous thin film using a Gardnerblade. The glass substrate and film were kept in a leveled oven at a 55°C. for two days to remove the solvents. The dried film was then detachedfrom the glass substrate by immersing it in approximately 4 L water bathand then kept in the water bath overnight to ensure the removal ofsolvent residual (NMP and HPMI) by solvent exchange. The pristine PANIfilms were then dried in air for 1 hour, then sandwiched between twoKimWipes, and placed under a flat metal weight (15 kg) for one day tokeep the dry film flat. The resulting thermally cured, dense PANI filmhas a thickness of 1 μm-350 μm (as measured by micrometer or verniercalipers) depending on the thickness setting of the Gardner blade. Thefilms were then doped by immersing them into an acid solution (typicallyin the concentration range from 0.01M-0.25 M) for 24-144 hours followedby three rinses in 100 mL of purified water for 1-30 minutes.Metal Nano Structures on PANI: The acid doped films were cut into 5×5 mmpieces and immersed into freshly prepared aqueous metal ion solutions(AgNO₃, or HAuCl₄ plus HCl) after which, metallic nanostructuresimmediately and spontaneously begin to form on the PANT surface. Rinsingthe PANI film in water for 5 seconds halts all metal growth, and themetal structures are finally dried in air. Reaction conditions such asthe PANT film thickness, the concentration of metal ion, and the totalexposure time of the PANI film to the ion solution were all found todramatically affect the resulting metal nanostructures.

Example 1

FIGS. 1-4 contains representative SEM images of silver nanostructuresformed on a 350 μm thick citric acid doped PANT film and SERS spectra ofsilver nanostructures grown at each respective concentration. The silverstructures were grown from silver nitrate solutions with concentrationsof 1 mM (FIG. 1), 10 mM (FIG. 2), 20 mM (FIG. 3), and 100 mM (FIG. 4).The morphology varies with increasing silver ion concentration rangesfrom featureless particles to nanosheet assembly to rod-like assemblies.All of these silver nanoparticles exhibit strong SERS activitymanifested by the SERS spectra of mercaptobenzoic acid (MBA). The SERSsignal is collected under 1 mW of laser power and a collection time of10 secs.

Example 2

FIGS. 5-10 contain representative SEM images of silver nanostructuresformed on a 150 μm PANI film which are doped with various acids having apH of 1.5, and SERS spectra of silver nanostructures grown at 20 mM. ThePANT film were doped in tartaric acid (FIG. 5), phosphoric acid (FIG.6), camphosulfonic acid (FIG. 7), mandelic acid (FIG. 8), p-toluenesulfonic acid (FIG. 9), and poly(styrene sulfonic acid) (FIG. 10). Themorphology of silver particles grown on PANT films is acid dependent.The morphology ranges from particles to wires. All of these silvernanoparticles exhibit SERS activity manifested by the SERS spectra ofmercaptobenzoic acid (MBA). The SERS signal is collected under 1 mW oflaser power and a collection time of 10 secs.

Example 3

FIG. 11 shows representative SEM images of gold nanostructures formed ona 150 μm PANI film. The gold structures were grown for overnight from amixture, 2 mL of 200 mM AuCl₃ solution with 50 μL of 37% hydrochloricacid. The gold structure has some sharp edges. This Au particles exhibitSERS activity manifested by the SERS spectra of mercaptobenzoic acid(MBA). The SERS signal is collected under 1 mW of laser power and acollection time of 10 secs.

Example 4

The Ag particles grown on PANI films with different thickness rangesfrom 50 to 340 μm also show variation in morphology and they also showSERS activity that appears to be somewhat morphological dependent. Thisresult suggests another way of controlling the particle morphology aswell as their corresponding SERS activity.

Example 5

Ag particles were also grown on 150 μm thick undoped PANI films thatwere immersed in 1 ml of AgNO₃ solution (10 mM) containing a dopant (50μl of citric acid-0.25M). This simultaneous doping and depositionprocess results in Ag metal nanostructures very similar to thoseobserved with pre-doped PANI films. This co-doping/deposition method isgeneral, and applies to any dopant of choice.

1. A method of forming a metal-polymer composite comprising: a)providing a thermally-cured dense polyaniline substrate; b) contactingthe thermally-cured dense polyaniline substrate with an acid dopant toform a doped substrate; c) providing a solution comprising a metal ion;and d) contacting the solution comprising a metal ion with the dopedsubstrate, whereupon metal nanostructure deposits are formed on thedoped substrate, wherein the deposits have a morphology which isdependent upon the acid dopant.
 2. The method of claim 1, wherein themetal is selected from the group consisting of silver and gold.
 3. Themethod of claim 1, wherein the acid dopant is selected from the groupconsisting of tartaric acid, camphosulfonic acid, phosphoric acid,citric acid, mandelic acid, poly(styrene sulfonic acid),para-toluenesulfonic acid, and combinations thereof.
 4. The method ofclaim 1, wherein the nanostructure deposits are sheets, cubes, spheres,needles, rods, or wires.
 5. The method of claim 4, wherein thenanostructure deposits are wires.
 6. The method of claim 1, wherein themetal is silver and the acid dopant is citric acid.
 7. The method ofclaim 1, wherein the metal is silver, the acid dopant ispara-toluenesulfonic acid, and the nanostructure deposits are wires. 8.A method of forming a metal-polymer composite comprising: a) providing athermally-cured dense polyaniline substrate; b) providing a solutioncomprising a metal ion and an acid dopant; and c) contacting thesolution comprising the metal ion and acid dopant with thethermally-cured dense polyaniline substrate, whereupon metalnanostructure deposits are formed on the doped substrate, wherein thedeposits have a morphology which is dependent upon the acid dopant. 9.The method of claim 8, wherein the acid dopant is selected from thegroup consisting of tartaric acid, camphosulfonic acid, phosphoric acid,citric acid, mandelic acid, poly(styrene sulfonic acid),para-toluenesulfonic acid, and combinations thereof.
 10. The method ofclaim 8, wherein the nanostructure deposits are sheets, cubes, spheres,needles, rods, or wires.
 11. A method of performing surface-enhancedRaman spectral analysis of an analyte comprising: a) providing athermally-cured dense polyaniline substrate, wherein a surface of thesubstrate comprises metal nanostructure deposits and an acid dopant; b)bringing an analyte into effective contact with said metal-PANIcomposite; c) illuminating the metal-PANI composite with radiation of atleast one wavelength for causing elemental-state particles to produce aplasmon field and for causing said plasmon field to interact withmolecules of said analyte, in such a manner, so as to produce Ramanphotons; and, d) collecting scatted radiation emitted from saidmetal-PANI composite for spectral analysis.
 12. The method of claim 11,wherein the metal is selected from the group consisting of silver andgold.
 13. The method of claim 11, wherein the acid dopant is selectedfrom the group consisting of tartaric acid, camphosulfonic acid,phosphoric acid, citric acid, mandelic acid, poly(styrene sulfonicacid), para-toluenesulfonic acid, and combinations thereof.
 14. Themethod of claim 11, wherein the nanostructure deposits are sheets,cubes, spheres, needles, rods, or wires.
 15. A metal-polymer compositecomprising: a) a thermally-cured dense polyaniline substrate; b) an aciddopant; and, c) metal nanostructure deposits wherein the deposits have amorphology dependent upon the acid dopant.
 16. The metal-polymercomposite of claim 15, wherein the metal nanostructure deposits comprisea metal selected from the group consisting of silver and gold.
 17. Themetal-polymer composite of claim 15, wherein the acid dopant is selectedfrom the group consisting of tartaric acid, camphosulfonic acid,phosphoric acid, citric acid, mandelic acid, poly(styrene sulfonicacid), para-toluenesulfonic acid, and combinations thereof.
 18. Themetal-polymer composite of claim 15, wherein the metal nanostructuredeposits are sheets, cubes, spheres, needles, rods, or wires.
 19. Themetal-polymer composite of claim 15, wherein the nanostructure depositsare wires.
 20. The metal-polymer composite of claim 15, wherein themetal is silver and the acid dopant is citric acid.
 21. Themetal-polymer composite of claim 15, wherein the metal is silver, theacid dopant is para-toluenesulfonic acid, and the nanostructure depositsare wires.